Combined feedback and feed-forward linearization of rf power amplifiers

ABSTRACT

A power amplifier module and corresponding system are disclosed for linearizing the output from a power amplifier. Both a feedback system, containing a compensator and the power amplifier in a feedback loop, and a pre-distortion compensation system injecting pre-distortion signals into or before the feedback system are used to compensate for non-linearities in the overall system. The pre-distortion signals may be mixed with signals from the compensator or may be filtered to take into account the loop compensator transfer function of the feedback loop, mixed with baseband signals and then converted into analog signals that are provided to the feedback loop. In modules containing a tracking power supply, an envelope calculator calculates an RF envelope of the baseband signals, which the pre-distortion system uses in conjunction with the baseband signals to generate the pre-distortion signals mixed with the signals from the compensator.

TECHNICAL FIELD

The present application relates to radio frequency (RF) poweramplifiers. In particular, the application relates to the linearizationof RF power amplifiers.

BACKGROUND

In many types of electronic applications, especially those that containpower amplifiers, signal distortion plays a significant role. Oneparticular application in which a power amplifier is used is acommunication system, in which the power amplifier is used to increasethe signal strength of wireless transmissions between a base station anda wireless handset. In an ideal linear power amplifier, the ratio of theoutput power to the input power does not vary with the input power.However, communications systems are not ideal. Thus, the power amplifieris subject to nonlinearities that add noise and cause distortion.

There exist different techniques for improving linearization of poweramplifiers over different power levels. One of these techniques usesdigital pre-distortion. A pre-distortion system generally relies on apriori knowledge of the power amplifier characteristics to digitallypre-compensate for the power amplifier distortion. The signal iscorrected prior to being upconverted to radio frequency. Thepre-distortion tables can be generated and implemented in a variety ofknown ways. The values in the tables modify the signal to using theinverse characteristics of the power amplifier. However, to use thistechnique, the power amplifier distortion characteristics must be knownin advance to provide suitable linearization.

Another of these linearization techniques uses feedback. Using feedbacklinearization, the power amplifier is enclosed within a Cartesianfeedback system. Pre- and post-amplification signals are combined togenerate a corrected signal. While this approach does not rely onadvance knowledge of the power amplifier distortion characteristics, itmay be costly to implement. The pre-distortion and Cartesian feedbackapproaches are fundamentally different and not directly compatible. Forexample, feeding a closed Cartesian loop with the pre-distorted inputthat would be used in a basic pre-distortion system would produce grossdistortion since the Cartesian loop now tries to reproduce a referencesignal with considerable added distortion. Placing the pre-distortionsystem within the Cartesian loop is also undesirable since thepre-distortion system is best implemented digitally, leading to asignificant amount of processing delay, compromising the stability ofthe Cartesian feedback system. Thus, only one has been applied to agiven system at a time. In some instances, the linearization provided byeach technique alone has not been sufficient. It is thus desirable toprovide an enhanced linearization technique.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a graph of output signal for a power supply with and withoutpower supply modulation.

FIG. 2 is a first embodiment of a power amplifier module.

FIG. 3 is a second embodiment of a power amplifier module.

FIG. 4 is a third embodiment of a power amplifier module.

FIG. 5 is a simulation of non-modulated outputs of the power amplifiermodule of FIG. 2 and a power amplifier module with only feedbackcompensation.

FIG. 6 is a simulation of non-modulated I components of the poweramplifier modules of FIG. 5.

FIG. 7 is an enhanced view of the I component error signals shown inFIG. 5.

DETAILED DESCRIPTION

A linear power amplifier module and corresponding linearization systemis provided for a radio frequency (RF) power amplifier. The linear poweramplifier module may be used in radio transmitters of various types, forexample, those present in base stations or handsets. The combinedpre-distortion and feedback compensation system provides thesimultaneous benefits of both linearization mechanisms. Estimated I andQ error signals are generated and then injected into or before an analogfeedback loop so that the error is primarily compensated for directlyusing known or estimated characteristics of the power amplifier. Thiseffectively lowers the open-loop distortion of the power amplifier,thereby decreasing the closed-loop distortion when a feedback signal isapplied. The power amplifier module and method of linearization uses acombination of feedback-based linearization with pre-distortion/errorfeed-forward, where the error feed-forward is generated digitally andinjected such that there is no conflict between the feedback andfeed-forward error correction functions.

As indicated above, known techniques may not be sufficient to linearizecurrent power amplifier modules. In one particular example, theincorporation of power supply modulation in a power amplifier moduleadds a substantial amount of non-linearity. Power supply modulation isused to control the supply voltage provided to the power amplifier tocorrespond to the instantaneous output power level. Such a schememaximizes the efficiency of the power amplifier. For implementing powersupply modulation, an Ultra-Fast Tracking Power Supply (UFTPS) is usedto generate a rapidly-varying voltage to supply to the power amplifier.The power consumption of the power amplifier is reduced by using such avariable supply voltage. However, use of the UFTPS introduces asignificant amount of extra distortion at the output of the poweramplifier throughout the output signal spectrum. This is shown in thegraph of FIG. 1, which illustrates the output spectra of a 50 kHz inputsignal for a high-power power supply used in the current generation ofTetra base station. An ideal output (30) is shown, along with a measuredoutput using power supply modulation (10) and not using power supplymodulation (20). The modulation increases efficiency by 10% but alsogenerates an extra 10 dB of distortion near the carrier. The existinglinearization techniques used alone are not able to adequatelycompensate for UFTPS systems.

FIG. 2 illustrates one embodiment of the linear power amplifier module.In FIG. 2, the feed-forward error correction signals are digitallycalculated and injected after the feedback loop compensator. Thisreduces the distortion of the RF power amplifier before the feedbacklinearization is applied, thereby reducing the amount of correctioncompensated for by the feedback loop.

Specifically, the linear power amplifier module 200 in FIG. 2 contains abaseband generator 202, error tables 204, D/A converters 205, firstcombiners 206, loop compensator 208, second combiners 210, an I/Qmodulator 212, an I/Q demodulator 214, and a power amplifier 216. Thebaseband generator 202 and error tables 204 operate in the digitaldomain, while the first combiners 206, loop compensators 208, secondcombiners 210, I/Q modulator 212, I/Q modulator 214, and power amplifier216 operate in the analog domain.

The baseband generator 202 generates a digital quadrature-amplitudemodulated (QAM) signal. The output of the baseband generator 202, a pairof quadrature reference signals, I_(ref) and Q_(ref), are supplied toeach error table 204. The error tables 204 generate estimations of theerror signals I_(err, est), Q_(err, est) for the quadrature referencesignals I_(ref), Q_(ref). The estimated error signals I_(err, est),Q_(err, est) are generated by means of a lookup table. The error tables204 provide a portion of the error correction in the power amplifiermodule 200 by providing a stored scalar that compensates for the poweramplifier non-linearity. The scalar is predetermined, e.g., using thevalue determined or estimated by the power amplifier manufacturer orusing a training sequence in the factory, to determine the initialnon-linearity of the power amplifier 216. A digital-to-analog converter205 converts each of the quadrature reference signals I_(ref), Q_(ref),and the estimated error signals I_(err,est), Q_(err,est) from a digitalsignal into an analog signal. As shown, four D/A converters 205 are usedin total. The signals passing through the D/A converters 205 arepre-compensated such that the group delay of all signals that passthrough D/A converters 205 are same. Filters associated with the D/Aconverters 205 are not shown. A filter (not shown) can also be placed inthe digital domain (i.e., in FIG. 2 to the left of the D/A converters205) to filter the signals from the error table.

Each of the quadrature reference signals I_(ref), Q_(ref) is alsosupplied to a different first combiner 206. The first combiner 206combines the quadrature reference signals I_(ref), Q_(ref) withquadrature feedback signals I_(out), Q_(out) to form error signalsI_(err), Q_(err). In a power modulator system with perfect errorcorrection, the error signals I_(err), Q_(err) would be non-existent.The error signals I_(err), Q_(err), are individually provided to a loopcompensator 208, that shapes the loop gain of the system. The errorsuppression of the loop compensator 208 is frequency dependent such thata larger amount of error suppression by the loop compensator 208 isprovided at lower frequencies, less at higher frequencies. Each of thecompensated signals I_(comp), Q_(comp) is then combined with thecorresponding estimated error signal I_(err, est), Q_(err, est) at thesecond combiner 210 to form modulator input signals I_(in), Q_(in). Themodulator input signals I_(in), Q_(out), which are at the basebandfrequency, are then supplied to a modulator 212. The modulator 212combines the modulator input signals I_(in), Q_(in) into a single signaland modulates the signal at the desired RF frequency. The RF signal issupplied to a power amplifier 216 and then the amplified RF signal issupplied to an antenna. The amplified RF signal is sampled by a coupler218 and the sampled signal is demodulated by the demodulator 214. Thedemodulator 214 generates the quadrature feedback signals I_(out),Q_(out) baseband, which are then supplied to the first combiners 206.

As above, in this embodiment, the feed-forward error correction signals(I_(err, est), Q_(err, est)) are digitally calculated, turned into ananalog signal, and injected after the feedback loop compensator 108. Asa portion of the correction for the power amplifier module 200 occursdue to the injected correction signals, the amount of correction thatthe feedback loop is to supply is correspondingly decreased.

In another embodiment, illustrated in FIG. 3, the pre-distortion isapplied in the digital domain. However, the errors generated by thepower amplifier and reduced by the feedback loop are taken into account.This is accomplished by (in addition to knowing or estimating the poweramplifier characteristics) inverting the loop compensator transferfunction in order to provide a filter that pre-compensates for the poweramplifier errors that are removed by the feedback loop.

Specifically, the linear power amplifier module 300 in FIG. 3 contains abaseband generator 302, error tables 304, D/A converters 305, filters320, first combiners 306, loop compensator 308, second combiners 310, anI/Q modulator 312, an I/Q demodulator 314, and a power amplifier 316.The baseband generator 302, error tables 304, and filter 320 and secondcombiners 310 operate in the digital domain, while the first combiners306, loop compensators 308, I/Q modulator 312, I/Q modulator 314, andpower amplifier 316 operate in the analog domain.

The baseband generator 302 generates a digital QAM signal. The output ofthe baseband generator 302, a pair of quadrature reference signals,I_(ref) and Q_(ref), are supplied to each error table 304. The errortables 304 generate estimations of the error signals I_(err, SS est),Q_(err, SS est) for the quadrature reference signals I_(ref), Q_(ref).The estimated error signals I_(err, SS est), Q_(err, SS est) aregenerated in a manner similar to the embodiment of FIG. 2. As describedabove, the estimated error signals I_(err, SS est), Q_(err, SS est) donot take into account the effects of the feedback loop. The errorestimates are smaller but more uncertain and less robust than that ofFIG. 2. One reason for this is because the loop compensator transferfunction of the feedback system is typically known with someuncertainty. Additionally, since the loop compensator is generally anintegrator-type element, error estimates will be high-pass filtered,increasing the amount of high-frequency energy in these signals. Thisgenerally makes it more difficult to accurately reproduce the I and Qreferences through an A to D converter. This may lead to more bitsand/or a higher sampling rate being used in the D/A converters 305.

The estimated error signals I_(err, SS est), Q_(err, SS est) are thensupplied to a digital filter 320. The filter 320 filters the estimatederror signals I_(err, SS est), Q_(err, SS est) to take into account thefeedback loop by providing the inverse loop compensator transferfunction and form final estimate error signals I_(err, est),Q_(err, est). Each of the quadrature reference signals I_(ref), Q_(ref)is supplied to one of the second combiners 310, where it is combinedwith the corresponding final estimate error signal I_(err, est),Q_(err, est) and converted by a D/A converter 305 to produce an analogoutput reference signal I_(ref; PD), Q_(ref; PD). Unlike the embodimentof FIG. 2, in FIG. 3 only two D/A converters are used as there are onlytwo digital signals to be converted to analog signals.

The analog output reference signals I_(ref; PD), Q_(ref; PD) aresupplied to first combiners 306 where each is combined with thecorresponding quadrature feedback signals I_(out), Q_(out) to form errorsignals I_(err), Q_(err). The error signals I_(err), Q_(err) areindividually provided to a loop compensator 308 to form modulator inputsignals I_(in), Q_(out). The modulator input signals I_(in), Q_(out),which are at the baseband frequency, are then supplied to a modulator312. The modulator 312 combines the modulator input signals I_(in),Q_(out) into a single signal and modulates this signal at the desired RFfrequency. The RF output signal is supplied to a power amplifier 316 andthen the amplified RF signal is supplied to an antenna (not shown). Theamplified RF signal is sampled by a coupler 318 and the sampled signalis demodulated by the demodulator 314. The demodulator 314 generates thequadrature feedback signals I_(out), Q_(out), which are then supplied tothe first combiners 306.

Another embodiment of a linear power amplifier module is shown in FIG.4. In this embodiment, a UFTPS is incorporated into the error estimationsystem. Since the power amplifier supply voltage substantiallyinfluences the power amplifier behavior, estimating this at digitalbaseband level (using a digital filter for modeling UFTPS behavior)allow for more precise estimation of the open-loop I and Q errorsgenerated by the PA.

Specifically, the linear power amplifier module 400 in FIG. 4 contains abaseband generator 402, error tables 404, first combiners 406, loopcompensator 408, second combiners 410, an I/Q modulator 412, an I/Qdemodulator 414, a power amplifier 416, a UFTPS model 420, and a UFTPS422. The baseband generator 402, error tables 404, and UFTPS model 420operate in the digital domain, while the first combiners 406, loopcompensators 408, second combiners 410, I/Q modulator 412, I/Q modulator414, power amplifier 416, and UFTPS 422 operate in the analog domain.

The baseband generator 402 generates a digital QAM signal. The basebandgenerator 402 calculates the RF envelope V_(env), which is followed bythe UFPTS 422 and filtered by the UFPTS model 420. As in FIG. 2, in theembodiment of FIG. 4, the output of the baseband generator 402, thequadrature reference signals, I_(ref) and Q_(ref), are supplied to eacherror table 404. The UFPTS model 420 also supplies a signal to each ofthe error tables 404. This signal is generated by the UFPTS model 420filtering the digital RF envelope V_(env). The error tables 404 generateestimations of the error signals I_(err, est), Q_(err, est) for thequadrature reference signals I_(ref), Q_(ref) based on both thequadrature reference signals, I_(ref) and Q_(ref) and the signals fromthe UFPTS model 420. As in other embodiments, D/A converters (not shown)convert each of the RF envelope V_(env), the quadrature referencesignals I_(ref), Q_(ref), and the estimated error signals I_(err, est),Q_(err, est) from a digital signal into an analog signal. The transitionbetween analog and digital signals is shown in FIG. 4 as the dottedvertical line. Thus, in this embodiment, five D/A converters are used intotal.

Each of the quadrature reference signals I_(ref), Q_(ref) is supplied toa different first combiner 406. The first combiner 406 combines thequadrature reference signals I_(ref), Q_(ref) with quadrature feedbacksignals I_(out), Q_(out) to form error signals I_(err), Q_(err). Theerror signals I_(err), Q_(err) are individually provided to a loopcompensator 408. Each of the compensated signals I_(comp), Q_(comp) isthen combined with the corresponding estimated error signalI_(err, est), Q_(err, est) at the second combiner 410 to form modulatorinput signals I_(in), Q_(in). The modulator input signals I_(in),Q_(in), which are at the baseband frequency, are then supplied to amodulator 412. The modulator 412 combines the modulator input signalsI_(in), Q_(in) into a single signal and modulates the signal at thedesired RF frequency.

The RF output signal from the modulator 412 is supplied to a poweramplifier 416. The RF envelope V_(env) is supplied to the UFTPS 420. TheUFTPS 420 supplies a supply voltage V_(s,PA) to the PA 416. This supplyvoltage V_(s,PA) is dependent on the envelope of the reference signalsI_(ref), Q_(ref). The amplification of the PA 416 is dependent on thesupply voltage V_(s,PA), and thus the amplified RF signal from the PA416 is dependent on the supply voltage V_(s,PA). The amplified RF signalfrom the PA 416 is then supplied to an antenna (not shown). Theamplified RF signal is sampled by a coupler 418 and the sampled signalis demodulated by the demodulator 414. The demodulator 414 generates thequadrature feedback signals I_(out), Q_(out), which are then supplied tothe first combiners 406.

As illustrated, in this embodiment, the UFTPS is incorporated into theerror estimation system. The UFTPS may also be incorporated into theerror estimation system shown in FIG. 4. Since the power amplifiersupply voltage substantially influences the power amplifier behavior,estimating this behavior at the digital baseband level (using a digitalfilter for modeling the UFTPS behavior) allows for more preciseestimation of the open-loop I and Q errors generated by the poweramplifier.

This is shown in the simulations shown in FIGS. 5-7. These figuresillustrate various outputs for the power amplifier module shown in FIG.2 and a power amplifier module that has feedback compensation but notpre-distortion compensation. The signals in the power amplifier modulehave not been simulated to be modulated by the I/Q modulator 212 orde-modulated by the I/Q demodulator 214. As can be seen, for frequenciesrelatively close (e.g., about 30-50 kHz) to the baseband frequency thecombination of the compensation techniques results in about a 3 dBimprovement over the use of feedback compensation alone. For frequenciesfarther away (e.g., over about 150 kHz), the combination of thecompensation techniques results in about a 6 dB improvement over the useof feedback compensation alone.

Turning to FIG. 6, the individual I components in the power amplifiermodule have been simulated and shown in a plot of voltage (in V) vs.time (in s). Specifically, the output I component from the coupler 218(I_(out)) and the I error component from the first combiner 206(I_(err)) for both power amplifier modules and the analog I estimatederror (I_(est, err)) from the error table 204 for the power amplifiermodule containing predistortion compensation are shown. As can be seenin FIG. 6, the difference between the output signals (I_(out)) isrelatively small and not easily seen on the voltage scale provided forthe y-axis. Even on the scale in FIG. 6, it is clear that the actualerror signal (I_(err)) supplied to the loop compensator 208 is differentbetween the power amplifier modules. Specifically, as can be seen bothin FIG. 6 and in the blowup of FIG. 7, the error signal (I_(err)) issubstantially smaller over a large portion of the time for the poweramplifier module that contains both types of compensation than for thepower amplifier module that contains only feedback compensation.

Although the embodiments describe a Cartesian feedback loop (a feedbackloop using analog Cartesian signals, the embodiments could equally wellemploy a polar (or other) feedback loop.

It will be understood that the terms and expressions used herein havethe ordinary meaning as is accorded to such terms and expressions withrespect to their corresponding respective areas of inquiry and studyexcept where specific meanings have otherwise been set forth herein.Relational terms such as first and second and the like may be usedsolely to distinguish one entity or action from another entity or actionwithout necessarily requiring or implying any actual such relationshipor order between such entities or actions. The terms “comprises,”“comprising,” or any other variation thereof, are intended to cover anon-exclusive inclusion, such that a process, method, article, orapparatus that comprises a list of elements does not include only thoseelements but may include other elements not expressly listed or inherentto such process, method, article, or apparatus. An element proceeded by“comprises . . . a” does not, without more constraints, preclude theexistence of additional identical elements in the process, method,article, or apparatus that comprises the element.

Those skilled in the art will recognize that a wide variety ofmodifications, alterations, and combinations can be made with respect tothe above described embodiments without departing from the spirit andscope of the invention defined by the claims, and that suchmodifications, alterations, and combinations are to be viewed as beingwithin the purview of the inventive concept. Thus, the specification andfigures are to be regarded in an illustrative rather than a restrictivesense, and all such modifications are intended to be included within thescope of present invention. The benefits, advantages, solutions toproblems, and any element(s) that may cause any benefit, advantage, orsolution to occur or become more pronounced are not to be construed as acritical, required, or essential features or elements of any or all theclaims. The invention is defined solely by the appended claims includingany amendments made during the pendency of this application and allequivalents of those claims as issued.

1. A power amplifier module comprising: a feedback system containing acompensator and a power amplifier in a feedback loop; and apre-distortion compensation system compensating for signals incooperation with the feedback system, the pre-distortion compensationsystem injecting pre-distortion signals into or before the feedbacksystem, the pre-distortion signals compensating for non-linearity of thepower amplifier.
 2. The power amplifier module of claim 1, wherein thefeedback system is an analog system in which all signals passing throughcomponents in the feedback system are analog signals and thepre-distortion system is a digital system in which all signals passingthrough components in the pre-distortion system are analog signals, thepower amplifier module further comprising a D/A converter for allsignals passing between the feedback system and the pre-distortionsystem.
 3. The power amplifier module of claim 2, further comprising abaseband generator generating quadrature reference signals, wherein: thefeedback loop comprises: first combiners to which demodulated quadraturesignals sampled from an antenna are supplied, loop compensatorsconnected to the first combiners such that signals from the firstcombiners are supplied to the loop compensators, the loop compensatorsshaping a loop gain of the feedback loop, and second combiners; and thepre-distortion compensation system comprises error tables connected withthe baseband generator, the error tables providing estimated errorsignals based on the quadrature reference signals and that compensatefor the power amplifier non-linearity, the error tables connected withthe second combiners.
 4. The power amplifier module of claim 3, wherein:the second combiners are connected to the loop compensators such thatsignals from the loop compensators are supplied to the second combiners,and analog signals dependent on the digital estimated error signals aresupplied to the second combiners.
 5. The power amplifier module of claim4, wherein the baseband generator comprises an envelope calculator thatcalculates an RF envelope of the quadrature reference signals, the poweramplifier module further comprising a tracking power supply connected tothe baseband generator to which the RF envelope is supplied, thetracking power supply connected to the power amplifier, the trackingpower supply providing a supply voltage that depends on the RF envelopeto the power amplifier.
 6. The power amplifier module of claim 5,wherein the power amplifier module further comprises a tracking powersupply model filtering the RF envelope and supplying the filtered RFenvelope to the error tables, the estimated error signals based on thefiltered RF envelope in addition to the quadrature reference signals. 7.The power amplifier module of claim 3, the pre-distortion system furthercomprises a filter disposed between each error table and correspondingsecond combiner, the filter filtering the estimated error signal toprovide an inversion of a loop compensator transfer function of thefeedback loop, wherein the second combiners are connected between thebaseband generator and the first combiners, the second combinersconnected with the filter such that signals from the baseband generatorand signals from the filter are supplied to the second combiners.
 8. Amethod of providing linearization for a power amplifier, the methodcomprising: providing feedback using a feedback loop to partiallycompensate for non-linearity of the power amplifier; and providingpre-distortion to additionally compensate for the non-linearity of thepower amplifier in cooperation with the feedback, the pre-distortioninjecting pre-distortion signals into or before the feedback loop. 9.The method of claim 8, further comprising converting signals fromdigital signals to analog signals such that the signals provided withthe pre-distortion are digital and the signals provided with thefeedback are analog.
 10. The method of claim 9, wherein providing thepre-distortion comprises providing estimated error signals based onreference signals, analog conversions of the estimated error signalsbeing supplied to combiners in the feedback loop connected to loopcompensators in the feedback loop.
 11. The method of claim 10, furthercomprising: calculating an RF envelope of the reference signals; andproviding a supply voltage to the power amplifier that depends on the RFenvelope.
 12. The method of claim 11, further comprising filtering theRF envelope and supplying the filtered RF envelope to error tables thatprovide the estimated error signals such that the estimated errorsignals are based on the filtered RF envelope in addition to thereference signals.
 13. The method of claim 9, further comprisingfiltering the estimated error signal to provide an inversion of a loopcompensator transfer function of the feedback loop, and providing thepre-distortion comprising providing reference signals and the inversionto combiners prior to converting signals from the combiners into analogsignals.